The demand for human organs and tissues suitable for transplantation is steadily increasing. The gap between the demand and the availability of organs is very likely to grow even wider in view of the continuing improvements in transplantation procedures and outcome. A potential answer to this problem is the use of non-human primate organs. However, these animals are in short supply worldwide, and are considered to be endangered species.
The pig is a potential donor because of compatible physiological characteristics [Cooper, D. K. C., et al., Xenotransplantation, Springer-Verlag:481-500, 1991; Tumbleson, M. E. (ed.), Swine in Biomedical Research, Volume 3 (Plenum Press, New York, 1985); Stanton, H. C., et al. (eds.), Swine in Cardiovascular Research, Volumes I-III (CRC Press, Inc., Boca Raton, 1986)], size, breeding properties and maintenance costs. Organ transplantation between widely disparate species such as pig and man, however, is followed by antibody-mediated hyperacute rejection within minutes. If the problem of antibody-mediated rejection could be overcome, then pig to human xenotransplantation might be possible. If the hyperacute rejection is prevented, a pig organ transplanted into a human recipient may survive for a longer period than would be otherwise possible due to the "accommodation" [Bach, F. H., et al., Transplant. Proc., 23:205, 1991] of the organ and to the therapeutic suppression of cellular rejection [Simpson, M. A., et al., in Hardy, M. A. (ed.), Xenograft 25, 273-284 (Elsevier New York 1989); Michler, R. E., et al., Transplantation, 44(5): 632-636, 1987].
Antibody-mediated rejection of pig tissue
Human natural or preformed antibodies play a significant role in hyperacute rejection of pig organs [Welsh, K. I., et al., in Cooper, D. K. C., et al. (eds.), Xenotransplantation, 501-510 (Springer-Verlag 1991); Paul, L. C., in Cooper, D. K. C., et al. (eds.), Xenotransplantation, 47-67 (Springer-Verlag 1991); Platt, J. L., et al., in Cooper, D. K. C., et al. (eds.), Xenotransplantation, 69-79 (Springer-Verlag 1991); Somerville, C. A., et al., Kidney Int., 44, Suppl. 42: S112-S121, 1993]. Recent experiments strongly indicate that human natural antibodies directed against the .alpha.Gal(1.fwdarw.3).beta.Gal carbohydrate structure, referred to herein as "anti-gal antibodies", are the major factor in the hyperacute rejection of pig cells and organs [Good, A. H., Cooper, D. K. C., et al., Transplant Proc., 24(2):559-652, 1992]. These antibodies bind to pig cells, which, like cells of most subprimate mammals, express .alpha.Gal(1.fwdarw.3)Gal on their surfaces. Anti-gal antibodies bind to the cell surface, activate the complement system, and thus cause cell damage.
in one approach, described in U.S. Ser. No. 08/049,817, filed Apr. 20, 1993, by D. K. C. Cooper and E. Koren, entitled, "Genetically Engineered Animals for Use as Organ Donors," a genetically-engineered animal, such as a pig, which is deficient in the .alpha. 1.fwdarw.3 galactosyl transferase gene, resulting in non-expression of galactosyl epitopes on its organs and tissues is constructed. However, this method does not address the problem of hyperacute rejection at its source: the immunologic response by the recipient to the transplanted tissue.
In a second approach, hyperacute rejection can be inhibited by the addition of oligosaccharides with terminal .alpha.Gal(1.fwdarw.3).beta.Gal residues to human plasma, which competitively inhibit binding of these naturally occurring antibodies to the xenograft. Alternatively, the same .alpha.Gal(1.fwdarw.3).beta.Gal containing oligosaccharides immobilized to a solid support can be used as immunoaffinity adsorbers to remove anti-gal antibodies from the blood. These two approaches can be applied in complementary fashion with even better prospects for successful prevention of hyperacute rejection of pig organs. U.S. Ser. No. 07/933,466, filed Aug. 21, 1992, by Good, et al., discloses the use of both immunoaffinity adsorbers and parenteral administration of .alpha.Gal(1.fwdarw.3).beta.Gal containing oligosaccharides to inhibit hyperacute rejection of pig organs.
However, anti-gal antibodies bind to .alpha.-gal oligosaccharides with relatively low affinity. This could necessitate a high concentration of oligosaccharides in the transplant recipient's blood in order to block the binding of circulating antibodies to the transplanted organ. This could result in side effects due to the high concentrations of carbohydrate. Moreover, the carbohydrate is relatively expensive to make, and treatment would be expensive. The low binding affinity could also have an adverse impact on extracorporeal immunoaffinity treatment by making the removal of anti-gal antibodies relatively inefficient.
Furthermore, anti-gal antibodies, although important in xenograft rejection, may not be the only human anti-pig antibodies responsible for hyperacute rejection. Anti-pig antibodies that bind to the protein components on the surface of pig cells have also been reported [Tuso, P. J., et al., Presentation at the American Society of Transplant Surgeons, 12th Annual Meeting in Houston, May 17-19, 1993]. In addition, individual differences in profiles of anti-pig antibodies may exist among potential recipients. Some patients are likely to have less anti-gal antibodies, and more of the "non anti-gal" antibodies.
It is therefore an object of the present invention to provide a method for inhibiting antibody-mediated rejection of xenografts in human patients.
It is a further object of the present invention to provide a means for depleting anti-xenotransplant antibodies from human blood.
It is another object of the present invention to provide a means for quantification and suppression in transplant recipients of lymphocytes involved in production of anti-xenotransplant antibodies.
It is another object of the invention to provide a means for predicting the severity of xenotransplant rejection in human recipients.